crimson: coroutinize SegmentManager::get_segment_manager()

crimson: coroutinize SegmentManager::get_segment_manager()

this change was inspired by following warning:

```
[1/3] Building CXX object src/crimson/os/seastore/CMakeFiles/crimson-seastore.dir/segment_manager.cc.o
/home/kefu/dev/ceph/src/crimson/os/seastore/segment_manager.cc:45:15: warning: lambda capture 'FNAME' is not used [-Wunused-lambda-capture]
   45 |       ).then([FNAME,
      |               ^
```

but we went further by coroutinize the whole method. because the return
value of ioctl() is not checked before this change, and clang correctly
flagged this with a warning, we marker it with `[[maybe_unused]]`, we
will fix it in a separate change.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>